Considerable interest has recently been focused on the problem of limiting the mass of particulate matter emitted with the exhaust gases from diesel and other internal combustion engines. In the case of diesel engines, a great deal of effort is currently being expended to develop practical and efficient devices and method for reducing emission of particulates in exhaust gases.
One method for accomplishing this is to provide a suitable particulate trap in the exhaust system of a diesel engine, the trap having at least one filter positioned therein which is capable of efficiently trapping the particulates from the exhaust gases and also being adapted to be regenerated as by the in-place incineration of the trapped particulates.
A ceramic wall-flow monolith particulate filter of the type disclosed, for example, in U.S. Pat. No. 4,276,071 entitled "Ceramic Filters For Diesel Exhaust Particulates", issued June 30, 1981 to Robert J. Outland, has emerged as a preferred form of such a filter device.
Such a ceramic wall-flow monolith particulate filter includes an outer wall interconnected by a large number of interlaced thin porous internal walls which define a honeycomb structure to provide parallel channels running the length thereof. Alternate cell channel openings on the monolith face are blocked and, at the opposite end the alternate channel openings are blocked in a similar manner but displaced by one cell whereby to define inlet channels and outlet channels.
With this filter arrangement, the exhaust gas cannot flow directly through a given inlet channel but is forced to flow through the separating porous walls into an adjacent outlet channel. The exhaust gas is thus filtered as it flows through the porous walls between adjacent channels.
As this type ceramic filter is presently manufactured, the ceramic walls thereof are fabricated by extrusion and then fired. After firing, the alternate channel openings are plugged in a suitable manner to provide the structure described hereinabove having a plurality of inlet channels and a plurality of outlet channels arranged in checkerboard fashion.
Because of this above-described method of construction of such a ceramic filter device, all known filter structures of the above-described type have the channels thereof of uniform cross-section throughout the longitudinal length of the filter element.
In operation, this type of filter device is situated in the engine exhaust system and removes particulates from the exhaust gases by trapping of the particulates on the walls of the inlet passages or channels separating them from the outlet channels. As will be apparent however, packaging considerations for the use of such a filter in a vehicle application restrict the size of such a filter device thus limiting its particulate storage capacity. Therefore, as used in a vehicle emission system, the accumulated soot must periodically be oxidized, as by incineration, to effect regeneration of the filter device.
However, during the incineration of accumulated particulates on a filter, the uncontrolled burning thereof can result in excessively high temperatures. Such high temperatures, if not evenly distributed throughout the body of the filter, can result in thermal gradients which may cause mechanical failure of the filter structure or, even worse, such high temperatures may actually exceed the melting temperature of the material used to fabricate the filter.
It has now been discovered that due to the parallel flow of exhaust gases through a uniformly porous wall with constant-area inlet and outlet channels, the axial distribution of particulate loading is in fact not uniform and will, in effect, be identical to that of the transverse velocity of the exhaust gas flowing through the porous wall. This is due to the fact that diesel exhaust particulates are sufficiently small to follow the exhaust gas flow almost exactly, and therefore, the axial distribution of particulate loading will be identical to that of the transverse velocity, that is, increasingly heavy towards the downstream end of the inlet channels.